In the search for subsurface hydrocarbons, once a borehole has been drilled, it is nearly always necessary to perform an evaluation of the properties of the rock formation in which the borehole has been formed. The most common purpose for performing such an evaluation is to determine the water saturation and porosity of a porous formation. This information is clearly essential in order to properly evaluate the economics of converting the open borehole into a producing well and in order to develop a plan for producing the hydrocarbon fluids from the hydrocarbon bearing formations if the economic evaluation is favorable.
The technique that has achieved greatest acceptance in the oil industry for determining water saturation and porosity is that which has become to be known in the industry as wireline logging. The vast majority of the wells which are evaluated by wireline conveyed instruments in order to determine water saturation and porosity are logged by a suite of logs commonly referred to as the triple combo. The triple combo ordinarily includes three wireline logs including a resistivity log (either an electrical current measurement or an electrical induction measurement) for measuring formation resistivity, a neutron measurement for measuring formation porosity, and a gamma ray measurement for measuring formation density. In addition, sometimes the triple combo will include a sonic log.
While the results that are derived from wireline techniques for the most part are satisfactory, it is becoming increasingly recognized by those skilled in the art that there may be advantages to be derived from non-wireline techniques. Unfortunately, wireline logging must be performed after the formation of the borehole is complete. Thus, wireline logging may occur hours or even days after the borehole has been formed. As a result, the formation and the borehole may change with time so that the results to be obtained by the wireline investigations may be less than optimal and the formation properties of greatest interest may be obscured.
For example, the electrical properties of the formation may change over time as the drilling fluid seeps into or invades the formation. It is known that as the process of invasion occurs, the drilling fluid separates into a filtrate which penetrates the formation and a filtercake formed of the solids in the drilling fluid, which coats the surface of the borehole. The invasion modifies the readings obtainable by, for example the resistivity tool, while the filtercake makes it difficult for a wireline instrument to be placed directly against the rock so that a tool "standoff" condition results. "Standoff" has an adverse effect on many wireline instruments including the neutron porosity instrument.
Another adverse effect that may occur over time is that the formation itself may change. The formation material may swell when exposed to the drilling fluid thereby closing off the borehole. On occasion the formation matrix may be so insubstantial as to sluff off or collapse into the borehole to create a "cave" which poses standoff difficulties for many of the wireline investigations. If the tendency of the formation material to collapse into the borehole is extreme, the borehole itself may collapse or bridge over which not only may prevent further wireline logging, but also may result in lost tools and/or a lost well.
Additionally, the time required to complete the wireline investigation delays the process of converting the borehole into a producing well at significant expense.
It is therefore clear that if the measurements of the formation properties typically made after the completion of the borehole by the wireline technique could be performed while the well is being drilled, many of the above difficulties inherent in wireline logging would be avoided.
The satisfactory performance of formation evaluation during the course of excavating the borehole is only now becoming available to the oil industry. Previously, techniques had not been developed that satisfactorily addressed the problems inherent in formation evaluation while drilling. For example, the industry has prudently been reluctant to include a radioactive source in the drill string.